The present invention relates to a printer, facsimile apparatus or similar electrophotographic image forming apparatus and, more particularly, to an intermediate transfer type image forming apparatus which sequentially performs primary and secondary image transfer with the intermediary of an intermediate transfer medium implemented as, e.g., a belt, and the intermediate transfer member.
Today, electrophotographic image forming apparatuses having full-color copying and printing capabilities are extensively used. For the transfer of a full-color image to a recording medium, a so-called transfer drum type system and an intermediate transfer body, double transfer type system or simply intermediate transfer type system are available. The transfer drum type system sequentially forms a yellow (Y) image, magenta (M) image, cyan (C) image and black (BK) image on a photoconductive element or similar image carrier, and sequentially transfers them one above the other to a recording medium fixed on a transfer drum. The intermediate transfer type system sequentially transfers the Y, M, C and BK images from the image carrier to an intermediate transfer medium one above the other, and then collectively transfers the resulting full-color toner image to a recording medium. The intermediate transfer type system is advantageous over the transfer drum type system because it is paper-free and has a full-face copying ability.
However, the problem with the intermediate transfer type system is that toner is scattered on the intermediate transfer medium at the time of primary transfer. Specifically, when the toner image is transferred from the image carrier to the medium, it fails to reach an expected position on the medium and is scattered therearound. The resulting image is blurred. Particularly, such an image lacks sharpness when it comes to thin lines.
Some different approaches have heretofore been proposed to improve image quality in relation to the intermediate transfer type system, as follows.
(1) After toner having a high resistance has been non-electrostatically transferred to the intermediate transfer medium, a recording sheet is pressed against the medium by a heat roller (see Japanese Patent Laid-Open Publication No. 63-34570).
(2) After conductive toner has been non-electrostatically transferred to the intermediate transfer medium, a sheet is pressed against the medium by a heat roller (see Japanese Patent Laid-Open Publication No. 63- 34571).
(3) Every time a toner image is transferred to the intermediate transfer medium, the charge thereof is dissipated by a separation charger (see Japanese Patent Laid-Open Publication No. 1-282571).
(4) A higher transfer potential is assigned to the last transfer step than to the immediately preceding transfer step, and a preselected voltage is applied to the medium during the interval between consecutive transfer steps (see Japanese Patent Laid-Open Publication No. 2-183276).
(5) Means is provided for dissipating the charge of the intermediate transfer medium at a stage preceding means for transferring a toner image from the medium to a sheet (see Japanese Patent Laid-Open Publication No. 4-147170).
Among the above prior art approaches, the approaches (1) and (2) need a recording sheet to be pressed by the heat roller and cannot take advantage of the paper-free feature available with the intermediate transfer type system. The approaches (3) through (5) each needs exclusive discharging or voltage applying meas and/or control means for controlling it. This not only complicates a machine control mechanism, but also obstructs the miniaturization of the machine.